Wavelength division multiplexing (WDM) is commonly used in lightwave communications systems to provide increased transmission capacity. As is known to those skilled in the art, the addition of an optical add/drop capability in WDM-based systems provides added flexibility for removing and adding individual channels at intermediate nodes in the WDM transmission path, which further enhances the management of optical transmissions in lightwave communications systems. Although some advances have been made in the development of optical add/drop multiplexers (ADMs), there is still a need for a highly selective, loss-less ADM for use in WDM-based systems.
In general, most prior art ADMs utilize fixed or tunable fiber gratings to provide the necessary wavelength selectivity for the add/drop function. These prior art ADMs, whether of the fixed or tunable type, suffer numerous disadvantages, including: path loss for added, dropped, and "through" wavelengths; high implementation costs; and numerous design limitations.
Some prior art ADMs utilize tunable fiber gratings in conjunction with directional optical transfer devices, such as directional optical couplers with optical isolators, to accomplish wavelength-selective adding and dropping. Generally, these schemes employ fiber gratings that either pass through a desired wavelength or reflect a wavelength that is to be added or dropped. A major disadvantage of this type of ADM is the insertion loss associated with the splitting and/or combining of optical signals. Specifically, this type of ADM fails to effectively compensate for the losses that occur in the add, drop, and through paths.
Other prior art ADM schemes attempt to compensate for losses by utilizing optical circulators and fiber gratings in conjunction with a "complete" optical amplifier, commonly referred to as a "lumped" amplifier. This type of ADM typically includes fiber gratings disposed between a first and second optical circulator with a "lumped" amplifier at the input side of the first circulator. The lumped amplifier at the input side is able to provide gain for the optical signals that are dropped via the first circulator as well as those optical signals that pass through the ADM without being dropped. However, the optical signals that are added via the second circulator do not pass through the lumped amplifier. Consequently, this type of ADM does not effectively compensate for the insertion loss experienced by the optical signals in the add path of the ADM. Similarly, a lumped amplifier placed at the output side of the second circulator cannot effectively compensate for the insertion loss in the drop path because the optical signals reflected by the fiber gratings and dropped via the first circulator do not pass through the lumped amplifier at the output side of the second circulator. In sum, prior art ADMs with lumped optical amplifiers suffer the disadvantages of inefficient amplification, higher implementation costs, and added design complexity as more lumped amplifiers are added within the various output paths of the ADM.
Accordingly, there is a need for an optical add/drop multiplexer that is loss-less and that provides wavelength selective add/drop capability to overcome the shortcomings of the prior art.